The formation of polyacrylamide (PAM) in inverse microemulsion form was first reported by Speiser in 1976-1977 (J. Pharm. Sci 65(12), 1763(1976) and U.S. Pat. No. 4,021,364). In these publications, crosslinked, spherical PAM microemulsion particles with diameters less than 800 .ANG. were prepared at low solids and high surfactant concentrations to encapsulate biologically or pharmaceutically active material. Since then, numerous publications have appeared describing the preparation of PAM polymers via inverse microemulsion polymerization, i.e. U.S. Pat. Nos. 4,521,317, 4,681,912, G.B. 2,169,492A and WO 88/10274.
In general, these processes are either performed at low solids (10% or less) and high surfactant concentrations, or they are performed (1) in the presence of ionic monomers or (2) in the presence of added salts to attain higher solids (10-32%). PAM homopolymers have not been prepared at the higher solids, lower surfactants levels without the aid of stabilizing salts or ionic-comonomers.
U.S. Pat. No. 4,521,317 describes a low solids, high surfactant acrylamide homopolymer microemulsion polymerization using cationic or anionic surfactants. While this process is successful in preparing stable inverse PAM microemulsions, it requires large amounts of surfactants and oil and, consequently, is impractical commercially owing to its high inherent cost.
Higher solids, lower surfactant acrylamide/anionic copolymers and acrylamide/cationic copolymer microemulsions are also known. U.S. Pat. No. 4,681,912 describes a process for preparing (meth)acrylamide/(meth)acrylic acid copolymer microemulsion at higher polymer solids with less surfactant by choosing a nonionic surfactant or a nonionic surfactant mixture with the correct HLB in the correct proportions. WO 88/10274, and copending application Ser. No. 07/285,938 filed Dec. 19, 1988, now abandoned describe processes for preparing (meth)acrylamide/cationic copolymer microemulsions at higher polymer solids with less surfactant by choosing a nonionic surfactant or a nonionic surfactant mixture with the correct HLB in the correct proportions.
There are also several references which describe the preparation of higher solids, lower surfactant PAM homopolymers which can be prepared in the presence of added salts. G.B. 2,169,492A, for example, teaches that higher solids, lower surfactant PAM microemulsions can only be prepared if the microemulsion aqueous phase contains "at least one alkali metal salt of an aliphatic monocarboxylic acid, in a weight ratio of said alkali metal salt to said vinyl monomer of from 0.1/1 to 0.3/1". According to this reference, unstable PAM microemulsion latices result in the absence of salt. In addition, journal articles have also appeared which specifically point out that high solids PAM microemulsion homopolymers can not be prepared unless the AMD microemulsion polymerization is conducted in the presence of an added salt or an ionic monomer (see J. Colloid and Inter. Sci., 114 (2), 398 (1986); J. Colloid and Inter. Sci., 125(1), 97(1988); Colloid and Polymer Sci., 265, 1067(1987)).